U.S. patent number 7,038,150 [Application Number 10/886,142] was granted by the patent office on 2006-05-02 for micro environmental sensing device.
This patent grant is currently assigned to Sandia Corporation. Invention is credited to Laurance L. Lukens, Marc A. Polosky.
United States Patent |
7,038,150 |
Polosky , et al. |
May 2, 2006 |
Micro environmental sensing device
Abstract
A microelectromechanical (MEM) acceleration switch is disclosed
which includes a proof mass flexibly connected to a substrate, with
the proof mass being moveable in a direction substantially
perpendicular to the substrate in response to a sensed
acceleration. An electrode on the proof mass contacts one or more
electrodes located below the proof mass to provide a switch closure
in response to the sensed acceleration. Electrical latching of the
switch in the closed position is possible with an optional latching
electrode. The MEM acceleration switch, which has applications for
use as an environmental sensing device, can be fabricated using
micromachining.
Inventors: |
Polosky; Marc A. (Tijeras,
NM), Lukens; Laurance L. (Tijeras, NM) |
Assignee: |
Sandia Corporation
(Albuquerque, NM)
|
Family
ID: |
36216028 |
Appl.
No.: |
10/886,142 |
Filed: |
July 6, 2004 |
Current U.S.
Class: |
200/61.45R;
200/61.45M |
Current CPC
Class: |
H01H
1/0036 (20130101); H01H 35/14 (20130101); H01H
2001/0063 (20130101); H01H 2001/0084 (20130101) |
Current International
Class: |
H01H
35/02 (20060101) |
Field of
Search: |
;200/61.49,61.53,61.45R-61.45M
;73/514.01,514.16,488,514.21-514.24,514.31,514.34-514.36,514.38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
466021 |
|
Jan 1992 |
|
EP |
|
WO 8808613 |
|
Nov 1988 |
|
WO |
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Primary Examiner: Friedhofer; Michael
Assistant Examiner: Klaus; Lisa
Attorney, Agent or Firm: Hohimer; John P.
Government Interests
GOVERNMENT RIGHTS
This invention was made with Government support under Contract No.
DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The
Government has certain rights in the invention.
Claims
What is claimed is:
1. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass flexibly connected to the
substrate by a plurality of folded springs located around an outer
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular to the substrate in
response to a sensed acceleration, and further comprising a first
electrode located on a major surface of the proof mass; and (c) a
second electrode located proximate to the first electrode to
provide an electrical connection thereto upon movement of the first
electrode into contact with the second electrode in response to the
sensed acceleration.
2. The MEM acceleration switch of claim 1 wherein the substrate
comprises silicon.
3. The MEM acceleration switch of claim 2 wherein the proof mass
comprises silicon.
4. The MEM acceleration switch of claim 3 wherein each folded
spring comprises monocrystalline silicon.
5. The MEM acceleration switch of claim 3 wherein each folded
spring comprises polycrystalline silicon.
6. The MEM acceleration switch of claim 1 wherein the proof mass
has a thickness equal to or greater than the thickness of the
substrate.
7. The MEM acceleration switch of claim 1 wherein the major surface
of the proof mass has a shape that is circular or polygonal.
8. The MEM acceleration switch of claim 1 wherein the plurality of
folded springs comprises three to sixteen folded springs.
9. The MEM acceleration switch of claim 1 wherein each folded
spring comprises a first pair of spring arms connected to the proof
mass, and a second pair of spring arms connected to the substrate,
with the first and second pairs of spring arms being connected
together by a crossbeam.
10. The MEM acceleration switch of claim 1 wherein each folded
spring in the plurality of folded springs has a thickness in the
range of 1 50 microns.
11. The MEM acceleration switch of claim 1 wherein the first
electrode is electrically connected to the substrate through the
proof mass and the plurality of folded springs.
12. The MEM acceleration switch of claim 1 further comprising at
least one stop on the substrate extending over the periphery of the
proof mass to limit movement of the proof mass in a direction away
from the second electrode.
13. The MEM acceleration switch of claim 1 further comprising an
electrical latch to maintain the electrical connection between the
first and second electrodes after the sensed acceleration has
occurred.
14. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass flexibly connected to the
substrate by a plurality of folded springs located around a
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular to the substrate in
response to a sensed acceleration, and further comprising a first
electrode located on a major surface of the proof mass; and (c) a
second electrode located on a submount whereon the substrate is
attached with the second electrode being proximate to the first
electrode to provide an electrical connection thereto upon movement
of the first electrode into contact with the second electrode in
response to the sensed acceleration.
15. The MEM acceleration switch of claim 14 further comprising a
third electrode located on the submount, with the first electrode
upon contact with the second and third electrodes providing an
electrical connection between the second and third electrodes.
16. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass flexibly connected to the
substrate by a plurality of folded springs located around a
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular to the substrate in
response to a sensed acceleration, and further comprising a first
electrode located on a major surface of the proof mass; and (c) a
second electrode comprising a pin of a package whereon the
substrate is attached with the second electrode being located
proximate to the first electrode to provide an electrical
connection thereto upon movement of the first electrode into
contact with the second electrode in response to the sensed
acceleration.
17. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass flexibly anchored to the
substrate by a plurality of folded springs located around an outer
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular the substrate, and with the
proof mass having a metallization covering at least a portion of a
major surface thereof; and (c) at least one electrode located
proximate to the metallization to form an electrical contact
therewith upon movement of the proof mass in response to an
acceleration event above a threshold value.
18. The MEM acceleration switch of claim 17 wherein the substrate
and the proof mass each comprise silicon.
19. The MEM acceleration switch of claim 17 wherein the major
surface of the proof mass has a shape that is circular or
polygonal.
20. The MEM acceleration switch of claim 17 wherein the plurality
of folded springs comprises three to sixteen folded springs.
21. The MEM acceleration switch of claim 17 wherein each folded
spring comprises a first pair of spring arms connected to the proof
mass, and a second pair of spring arms connected to the substrate,
with the first and second pairs of spring arms being connected
together by a crossbeam.
22. The MEM acceleration switch of claim 17 wherein each folded
spring in the plurality of folded springs has a thickness in the
range of 1 50 microns.
23. The MEM acceleration switch of claim 17 further comprising an
electrical latch to maintain the electrical contact after
occurrence of the acceleration event.
24. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass flexibly anchored to the
substrate by a plurality of folded springs located around a
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular the substrate, and with the
proof mass having a metallization covering at least a portion of a
major surface thereof; and (c) at least one electrode located on a
submount or package whereon the substrate is attached with the at
least one electrode being located proximate to the metallization to
form an electrical contact therewith upon movement of the proof
mass in response to an acceleration event above a threshold
value.
25. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass formed, at least in part from the
substrate, with the proof mass being attached to the substrate by
three to sixteen springs located around an outer periphery of the
proof mass, and with the proof mass having a metallization covering
a majority of a major surface thereof and forming a first
electrode; and (c) a second electrode located beneath the proof
mass, with the first and second electrodes being spaced apart when
the proof mass is in a rest position, and with the first and second
electrodes being electrically connected together when the proof
mass is urged into contact with the second electrode in response to
an acceleration event directed substantially perpendicular to a
plane of the substrate and above a threshold value.
26. The MEM acceleration switch of claim 25 further comprising a
third electrode located beneath the proof mass, with the third
electrode being spaced apart from the first and second electrodes
when the proof mass is in the rest position, and with the third
electrode being electrically connected to the first and second
electrodes when the proof mass is urged into contact with the
second and third electrodes in response to the acceleration event
directed substantially perpendicular to a plane of the substrate
and above the threshold value.
27. The MEM acceleration switch of claim 26 wherein the second and
third electrodes are located on a submount or package whereon the
substrate is attached.
28. The MEM acceleration switch of claim 25 wherein the substrate
comprises silicon.
29. The MEM acceleration switch of claim 25 wherein each spring
comprises a folded spring.
30. The MEM acceleration switch of claim 25 wherein each spring
comprises an arcuate spring.
31. The MEM acceleration switch of claim 25 wherein each spring has
a thickness in the range of 1 50 microns.
32. The MEM acceleration switch of claim 25 further comprising an
electrical latch to maintain an electrical connection between the
first and second electrodes after occurrence of the acceleration
event.
33. A microelectromechanical (MEM) acceleration switch, comprising:
(a) a substrate; (b) a proof mass formed at least in part from the
substrate and attached to the substrate by a plurality of arcuate
springs located around an outer periphery of the proof mass, with
the proof mass forming a first electrode; and (c) a second
electrode located beneath the proof mass, with the first and second
electrodes being spaced apart when the proof mass is in a rest
position, and being electrically connected together when the proof
mass is urged into contact with the second electrode in response to
an acceleration event directed substantially perpendicular to a
plane of the substrate and above a threshold value.
34. The MEM acceleration switch of claim 33 further comprising an
electrical latch to maintain an electrical connection between the
first and second electrodes after occurrence of the acceleration
event.
35. The MEM acceleration switch of claim 33 further comprising a
plurality of lateral stops to limit movement of the proof mass in
the plane of the substrate.
Description
FIELD OF THE INVENTION
The present invention relates in general to microelectromechanical
(MEM) devices, and in particular to a MEM acceleration switch for
sensing when a particular level of acceleration or deceleration has
occurred.
BACKGROUND OF THE INVENTION
Acceleration switches can be used whenever a particular level of
acceleration or deceleration must be sensed. For example, in
automobiles, an acceleration switch can be used to sense a crash
and trigger the deployment of an airbag, or to sense or a severe
braking situation and trigger a seat belt tensioning device.
The present invention represents an advance in the art of
acceleration switches by providing an acceleration switch that can
be formed by micromachining. This minimizes a need for conventional
precision machining and piece-part assembly.
The acceleration switch of the present invention can be formed
using batch fabrication techniques, with individual devices having
acceleration set points which can be selected over a wide range
from 1 G to one thousand G or more, where G is the acceleration due
to gravity.
These and other advantages of the present invention will become
evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to a microelectromechanical (MEM)
acceleration switch which comprises a substrate; a proof mass
flexibly connected to the substrate by a plurality of folded
springs located around a periphery of the proof mass, with the
proof mass being moveable in a direction substantially
perpendicular to the substrate in response to a sensed
acceleration, and further comprising a first electrode located on a
major surface of the proof mass; and a second electrode located
proximate to the first electrode to provide an electrical
connection thereto upon movement of the first electrode into
contact with the second electrode in response to the sensed
acceleration. The MEM acceleration switch can further comprise an
electrical latch to maintain the electrical connection between the
first and second electrodes after the sensed acceleration has
occurred.
The substrate and the proof mass can each comprise silicon. Each
folded spring can also comprise silicon, which can be either
monocrystalline silicon when a silicon-on-insulator substrate is
used, or polycrystalline silicon when a silicon substrate is
used.
The thickness of the proof mass can be greater than or equal to the
thickness of the substrate; and each folded spring can have a
thickness in the range of 1 50 microns, for example, with the exact
thickness of each spring depending upon a threshold value of the
acceleration which is to be sensed by the MEM acceleration switch.
The major surface of the proof mass can have a shape that is
generally either circular or polygonal. The number of folded
springs can comprise, for example, three to sixteen folded springs.
Each folded spring can further comprise a first pair of spring arms
connected to the proof mass, and a second pair of spring arms
connected to the substrate, with the first and second pairs of
spring arms being connected together by a crossbeam. One or more
stops can be optionally provided on the substrate extending over
the periphery of the proof mass to limit movement of the proof mass
in a direction away from the second electrode.
The second electrode in the MEM acceleration switch can be located
on a submount whereon the substrate is attached, or alternately can
comprise a pin of a package whereon the substrate is attached. Yet
another electrode (i.e. a third electrode) can be located on the
submount or in the package. In this case, the first electrode can
contact the second and third electrodes as the proof mass is moved
in response to the sensed acceleration and thereby provide an
electrical connection (i.e. a switch closure) between the second
and third electrodes via the first electrode, or from the first
electrode to the second and third electrodes. The first electrode
can be electrically connected to the substrate through the proof
mass and the plurality of folded springs.
The present invention further relates to a MEM acceleration switch
which comprises a substrate; a proof mass flexibly anchored to the
substrate by a plurality of folded springs located around a
periphery of the proof mass, with the proof mass being moveable in
a direction substantially perpendicular the substrate, and with the
proof mass having a metallization covering at least a portion of a
major surface thereof; and at least one electrode located proximate
to the metallization to form an electrical contact therewith upon
movement of the proof mass in response to an acceleration event
above a threshold value. An electrical latch can optionally be
included in the MEM acceleration switch to maintain the electrical
contact after occurrence of the acceleration event.
The substrate and the proof mass can each comprise silicon. The
major surface of the proof mass can have a shape that is circular
or polygonal. One or more of the electrodes can be located on a
submount or package whereon the substrate is attached.
The plurality of folded springs generally comprises three to
sixteen folded springs, with each folded spring having a thickness,
for example, in the range of 1 50 microns (.mu.m). Each folded
spring comprises a first pair of spring arms connected to the proof
mass, and a second pair of spring arms connected to the substrate,
with the first and second pairs of spring arms being connected
together by a crossbeam.
The present invention also relates to a MEM acceleration switch
which comprises a substrate; a proof mass formed, at least in part
from the substrate, with the proof mass being attached to the
substrate by three to sixteen springs located around a periphery of
the proof mass, and with the proof mass having a metallization
covering a majority of a major surface thereof and forming a first
electrode; and a second electrode located beneath the proof mass,
with the first and second electrodes being spaced apart when the
proof mass is in a rest position, and with the first and second
electrodes being electrically connected together when the proof
mass is urged into contact with the second electrode in response to
an acceleration event directed substantially perpendicular to a
plane of the substrate and above a threshold value. The MEM
acceleration switch can further comprise an electrical latch to
maintain an electrical connection between the first and second
electrodes after occurrence of the acceleration event.
The substrate can comprise silicon. Each spring can comprise a
folded spring or an arcuate spring, and can have a thickness in the
range of 1 50 .mu.m, for example. The proof mass can have a
circular or polygonal shape. An optional third electrode can be
located beneath the proof mass, with the third electrode being
spaced apart from the first and second electrodes when the proof
mass is in the rest position, and with the third electrode being
electrically connected to the first and second electrodes when the
proof mass is urged into contact with the second and third
electrodes in response to the acceleration event directed
substantially perpendicular to a plane of the substrate and above
the threshold value. The second and third electrodes can be located
on a submount or package whereon the substrate is attached.
The present invention further relates to a MEM acceleration switch
which comprises a substrate; a proof mass formed at least in part
from the substrate and attached to the substrate by a plurality of
arcuate springs located around a periphery of the proof mass, with
the proof mass forming a first electrode; and a second electrode
located beneath the proof mass. The first and second electrodes are
spaced apart when the proof mass is in a rest position, and are
electrically connected together when the proof mass is urged into
contact with the second electrode in response to an acceleration
event that directed substantially perpendicular to a plane of the
substrate and above a threshold value. The MEM acceleration switch
can further comprise an electrical latch to maintain an electrical
connection between the first and second electrodes after occurrence
of the acceleration event. The MEM acceleration switch can also
comprise a plurality of lateral stops to limit movement of the
proof mass in the plane of the substrate.
Additional advantages and novel features of the invention will
become apparent to those skilled in the art upon examination of the
following detailed description thereof when considered in
conjunction with the accompanying drawings. The advantages of the
invention can be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a
part of the specification, illustrate several aspects of the
present invention and, together with the description, serve to
explain the principles of the invention. The drawings are only for
the purpose of illustrating preferred embodiments of the invention
and are not to be construed as limiting the invention. In the
drawings:
FIG. 1 shows a schematic plan view of a first example of the MEM
acceleration switch of the present invention.
FIGS. 2A and 2B show schematic cross-section views of the device of
FIG. 1 along the section line 1--1 in FIG. 1, with the device being
in a rest position (i.e. an open position) in FIG. 2A and in an
actuated position (i.e. a closed position) in FIG. 2B.
FIGS. 3A 3I show schematic cross-section views to illustrate
fabrication of the device of FIG. 1.
FIG. 4 shows a schematic plan view of a second example of the MEM
acceleration switch of the present invention.
FIG. 5 shows a schematic plan view of a submount used in the device
of FIG. 4.
FIGS. 6A and 6B show schematic cross-section views of the device of
FIG. 4 along the section line 2--2 in FIG. 4, with the device being
in the rest position in FIG. 6A and in the actuated position in
FIG. 6B.
FIGS. 7A 7G show schematic cross-section views to illustrate
fabrication of the device of FIG. 4.
FIG. 8 shows a schematic plan view of a third example of the MEM
acceleration switch of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown a schematic plan view of a
first example of a microelectromechanical (MEM) acceleration switch
10. The MEM acceleration switch 10 in FIG. 1 comprises a substrate
12, with a proof mass 14 being suspended from the substrate 12 by a
plurality of folded springs 16 so that the proof mass 14 is
flexibly connected (i.e. anchored) to the substrate 12 and can move
in a direction that is substantially perpendicular to the substrate
12 (i.e. substantially perpendicular to a plane formed by a major
surface of the substrate 12) in response to a sensed acceleration
(also termed herein an acceleration event, or an acceleration
component). The term "folded spring" as used herein refers to a
spring which comprises a plurality of spring arms arranged in
juxtaposition, with each adjacent pair of the spring arms being
connected together.
A plurality of stops 18 are also provided on the substrate 12 in
the MEM acceleration switch 10 of FIG. 1 to limit movement of the
proof mass 14 in an upward direction (i.e. out of the page of FIG.
1) so that any substantial motion of the proof mass 14 will be in a
downward direction (i.e. into the page of FIG. 1). Such downward
motion of the proof mass 14 will occur in response to an
upward-directed acceleration component to which the MEM
acceleration switch 10 is responsive.
FIGS. 2A and 2B show schematic cross-section views of the device 10
of FIG. 1 along the section line 1--1 in FIG. 1 to illustrate
operation of the MEM acceleration switch 10. In FIG. 2A, the device
10 is shown in a rest position without any applied acceleration
component. This is the position in which the MEM acceleration
switch 10 is fabricated. In the rest position, the proof mass 14 is
suspended from the substrate 12 by a plurality of folded springs 16
so that there is no contact between the proof mass 14 and an
underlying electrode 20.
In FIG. 2B, in response to an upward-directed acceleration
component A indicated by the arrow, the proof mass 14 is urged to
move in the opposite direction, and will contact the electrode 20
when the acceleration component A exceeds a predetermined threshold
value determined by the mass of the proof mass 14 and the stiffness
of the springs 16. In the example of FIGS. 1 and 2A, 2B, a
metallization 22 is provided on an underside major surface of the
proof mass 14 to form an electrode for the proof mass 14 which can
be electrically connected through the springs 16 to the substrate
12. The metallization 22, which covers a portion and preferably a
majority of the underside major surface of the proof mass 14, can
thus form a relatively low-resistance (e.g. less than a few hundred
Ohms, and preferably about 50 Ohms) electrical connection between
the proof mass 14 and the electrode 20 when the metallization 22 is
urged into contact with the electrode 20 in response to a sensed
acceleration above the threshold value.
In the example of the MEM acceleration switch 10 shown in FIGS. 1
and 2A, 2B, the substrate 12 with suspended proof mass 14 can be
attached to a submount or package 24 wherein one or more electrodes
20 are located. The form of attachment can be using a bonding
material 26 such as an adhesive (e.g. epoxy) or solder, or
alternately by directly bonding the substrate 12 to the submount or
package 24 (e.g. using an anodic wafer bonding process or a
low-temperature co-fired ceramic packaging process as known to the
art). Those skilled in the art will understand that there are many
different ways to attach the substrate 12 to a submount or package
24.
The package 24 can comprise a conventional semiconductor device
package such as a TO-5 or TO-18 header which can be sealed with a
cover (not shown) to form a hermetically-sealed MEM acceleration
switch 10. Alternately, the package 24 can be a custom-designed
package such as a hermetically-sealed low-temperature co-fired
ceramic (LTCC) package. Additional electronic circuitry can be
optionally included in a package 24 for the MEM acceleration switch
10 either adjacent to the switch 10 or formed on the substrate 12
when the substrate comprises silicon. This circuitry, which can
comprise a semiconductor integrated circuit as known to the art,
can be used, for example, to condition an electrical signal
produced by activation of the switch 10, or to provide a trigger
signal upon a closure of the switch 10 upon sensing a predetermined
acceleration event (i.e. an acceleration component A above the
threshold level).
Fabrication of the MEM acceleration switch 10 of FIGS. 1 and 2A, 2B
will now be described with reference to FIGS. 3A 3I. Although the
fabrication will be described in terms of micromachining based on
semiconductor processing steps including material deposition and
etching, those skilled in the art will understand that fabrication
of the switch 10 of the present invention can also be performed
using LIGA (an acronym based on the first letters for the German
words for lithography, electroplating and injection molding).
In FIG. 3A, a substrate 12 is provided which can be, for example, a
monocrystalline silicon substrate. The silicon substrate 12 is
preferably doped throughout for electrical conductivity (e.g. to
about 10.sup.18 cm.sup.-3).
In FIG. 3B, a layer of a sacrificial material 28 (e.g. silicon
dioxide or a silicate glass such as TEOS which can be deposited
from the decomposition of tetraethylortho silicate by low-pressure
chemical vapor deposition at about 750.degree. C. and densified by
a subsequent high temperature processing step) can be blanket
deposited over the silicon substrate 12 and patterned by reactive
ion etching. This patterning, done using a
photolithographically-defined etch mask which is not shown in FIG.
3B, removes portions of the sacrificial material 28 to form a mold
for a subsequent deposition of one or more layers of
polycrystalline silicon (also termed polysilicon) which will be
used to form the springs 16 and the stops 18 by surface
micromachining.
Those skilled in the art will understand that the references to
"patterning" and "patterned" herein refer to a series of process
steps which are well-known in the semiconductor device fabrication
art including applying a photoresist to the substrate 12, prebaking
the photoresist, aligning the substrate 12 with a photomask,
exposing the photoresist through the photomask, developing the
photoresist, baking the photoresist, etching away the surfaces not
protected by the photoresist, and stripping the protected areas of
the photoresist so that further processing can take place. The term
"patterning" can further include the formation of a hard mask (e.g.
comprising about 500 nanometers of TEOS) overlying a polysilicon or
sacrificial material layer in preparation for defining features
into the layer by anisotropic dry etching (e.g. reactive ion
etching).
In FIG. 3C, one or more layers of polysilicon 30 can be blanket
deposited over the substrate 12 to completely fill in the mold
formed by the patterned sacrificial material 28. The polysilicon 30
can then be planarized by a conventional chemical-mechanical
polishing step to provide a planar surface for the polysilicon 30
and the sacrificial material 28 as shown in FIG. 3C. This polishing
step can also be used to precisely control and adjust the thickness
of the polysilicon 30 at the locations wherein the folded springs
16 will be formed.
For this example of the present invention, each folded spring 16
comprises two pairs of spring arms, with one pair of spring arms
being connected between the proof mass 14 and a crossbeam 32, and
with the other pair of spring arms being connected between the
crossbeam 32 and the substrate 12 as shown in FIG. 1. The polishing
step allows the folded springs 16 to be precisely adjusted in
thickness to provide a predetermined threshold value of the
acceleration needed to activate the MEM acceleration switch 10. As
an example, the thickness of the folded springs 16 in the device 10
of FIG. 1, can be 3 .mu.m, with each spring arm having a length in
the range of 300 500 .mu.m and a width in the range of 10 30 .mu.m.
The crossbeam 32 can have the same thickness as the spring arms,
and can be 20 50 .mu.m wide and 200 500 .mu.m long.
In FIG. 3D, another layer of the sacrificial material 28 can be
blanket deposited over the substrate and patterned by reactive ion
etching to build up the mold for defining the shape of the various
elements being formed from the polysilicon 30 including the springs
16, stops 18 and a pair of supports 34 for attaching each spring 16
to the substrate 12 (see FIG. 1).
In FIG. 3E, another layer of the polysilicon 30 can be blanket
deposited over the substrate 12 to fill in the mold formed from the
sacrificial material 28, with any of the polysilicon 30 outside of
the mold being removed by another polishing step. This process can
be repeated, as needed, to build up the complete structure for the
springs 16, stops 18, and spring supports 34.
Each layer of polysilicon 30 described above can be deposited at a
temperature of about 580.degree. C. using low-pressure chemical
vapor deposition (LPCVD). The polysilicon 30 can be doped for
electrical conductivity (e.g. with phosphorous) to about the same
level as the substrate 12. This can be done either during
deposition of the polysilicon 30, or subsequently thereto using an
ion implantation or thermal diffusion step. Annealing of the
polysilicon 30 at a high temperature (e.g. about 1100.degree. C.
for a few hours) can be used to remove or reduce any residual
stress in the polysilicon 30.
In FIG. 3F, once the structure of the springs 16, spring supports
32 and the stops 18 has been built up by surface micromachining,
these elements can be left embedded in the sacrificial material 28;
and processing of a backside of the substrate 12 can be performed
to define the proof mass 14 which can be formed from at least
partially from the substrate 12 by a deep reactive ion etching
(DRIE) step. This DRIE process step can also be used to remove
portions of the substrate 12 underneath the springs 16 to provide
room for the springs 16 to move downward in response of an
acceleration-induced movement of the proof mass 14 (see FIG.
2B).
The DRIE process is disclosed in detail in U.S. Pat. No. 5,501,893
to Laermer, which is incorporated herein by reference. Briefly, the
DRIE process utilizes an iterative Inductively Coupled Plasma (ICP)
deposition and etch cycle wherein a polymer etch inhibitor is
conformally deposited as a film over the silicon substrate 12
during a deposition cycle and subsequently removed during an
etching cycle. The polymer film, which can be formed in a
C.sub.4F.sub.8/Ar-based plasma, deposits conformally over a
photolithographically patterned photoresist mask (not shown) which
is used to protect areas of the backside of the silicon substrate
12 not being etched and over sidewalls of a cavity 36 being etched
from the backside of the silicon substrate 12 around the proof mass
14 and underneath each spring 16. The cavity 36 can be, for
example, about 100 .mu.m wide around the proof mass 14 and can be
sized to be about 100 .mu.m larger than the lateral dimensions of
the springs 16.
During a subsequent etch cycle using an SF.sub.6/Ar-based plasma,
the polymer film is preferentially sputtered from the cavity 36 and
from the top of the photoresist mask. This exposes the silicon
substrate 12 in the region wherein the cavity 36 is being formed to
reactive fluorine atoms from the SF.sub.6/Ar-based plasma with the
fluorine atoms then being responsible for etching the exposed
portion of the silicon substrate 12. After the polymer at the
bottom of the cavity 46 has been sputtered away and the bottom
etched by the reactive fluorine atoms, but before the polymer on
the sidewalls of the cavity 36 has been completely removed, the
polymer deposition step using the C.sub.4F.sub.8/Ar-based plasma is
repeated. This cycle continues until a desired etch depth is
reached, which in the present case is completely through the
thickness of the substrate 12 to the sacrificial material 28, or
partway through the sacrificial material 28. Each polymer
deposition and etch cycle generally lasts only for a few seconds
(e.g. .ltoreq.10 seconds). The net result is that features can be
anisotropically etched into or completely through the silicon
substrate 12 while maintaining substantially straight sidewalls
(i.e. with little or no inward tapering).
The DRIE etching process can be used to form the proof mass 14 with
any desired shape for the major surfaces thereof including a
circular shape or a polygonal shape. These shapes for the major
surfaces produce a proof mass 14 in the form of a cylinder or right
polyhedron (i.e. prism), respectively. The proof mass 14 can have
lateral dimensions of up to a few millimeters and will generally be
about as thick as the substrate 12 (e.g. 400 600 .mu.m) or more
with the additional polysilicon 30 and metallization 22.
After the DRIE process step has been performed, the photoresist
mask can be stripped, and the substrate 12 cleaned. The sacrificial
material 28 can then be removed as shown in FIG. 3G. This can be
done by immersing the substrate 12 into a selective wet etchant
comprising hydrofluoric acid (HF) for up to a few hours to
selectively etch away the sacrificial material 28 while not
substantially chemically attacking the silicon substrate 12 or the
polysilicon 30 used to form the springs 16 and other elements of
the MEM acceleration switch 10.
The metallization 22, which has an overall thickness of up to a few
hundred nanometers, can be deposited over a lower major surface of
the proof mass 14 as shown in FIG. 3H. This can be done either
before or after die singulation when a plurality of MEM
acceleration switches 10 are batch fabricated on a common substrate
12. The metallization 22 and the electrode 20 are preferably formed
from nonoxidizable metals such as platinum and gold. The use of
dissimilar metals for the metallization 22 and electrode 20 is
beneficial to prevent adhesion which might otherwise result from
contact between the metallization 22 and electrode 20 when a single
type of metal (e.g. gold) is used for both. The adhesion of the
platinum or gold can be improved by the use of a thin layer of
titanium, or alternately a Ti/TiN/Ti adhesion stack which can be,
for example, about 120 nanometers thick. The metallization 22 can
be deposited over the lower surface of the proof mass 14 by
evaporation or sputtering through a shadow mask, with an exact area
of the metallization 22 depending upon the size and location of one
or more electrodes 20 which must be contacted by the metallization
22 for operation of the MEM acceleration switch 10.
In FIG. 3I, the substrate 12 can be attached to a submount or
package 24 to complete the MEM acceleration switch 10. As an
example, a conventional semiconductor device header (e.g. a TO-5 or
TO-18 header) can be used as a package 24 for the MEM acceleration
switch 10. Such a header 24 is generally formed from metal with one
or more electrodes 20 extending through the metal header and
electrically isolated by an insulating material 38 (e.g. glass or
ceramic) as shown in FIG. 3I. Each electrode 20 can be optionally
rounded slightly as shown in FIG. 3I. Other types of semiconductor
device headers comprising plastic or ceramic can also be used, in
which case, an insulating material 38 is not needed for electrical
isolation of the electrode(s) 20.
In FIG. 3I, the package 24 can include a recessed portion 40 to
allow for movement of the proof mass 14, or alternately a spacer 60
(see FIG. 6A) can be provided between the substrate 12 and the
package 24 to provide a predefined spacing between the
metallization 22 and the electrode 20 when the proof mass 14 is in
the rest position. This spacing can be, for example, 40 .mu.m and
will generally be in the range of 10 200 .mu.m or more.
As described previously, the substrate 12 and the submount or
package 24 can be permanently attached together with a bonding
material 26 such as an adhesive (e.g. a conductive epoxy) or
solder. In other embodiments of the present invention, the
substrate 12 can be attached to a submount or package using
diffusion bonding, low-temperature co-firing, etc. The form of
attachment between the substrate 12 and the submount or package 24
will, in general, depend on the materials used for the substrate 12
and the submount or package 24, and the magnitude of the
acceleration to be sensed, and cost and reliability factors. A lid
(not shown) can be provided over the substrate 12 and attached to
the substrate 12 or package 24 to form a hermetically-sealed device
10.
FIG. 4 schematically illustrates in plan view a second example of
the MEM acceleration switch 10 of the present invention. In the
device 10 of FIG. 4, a plurality of folded springs 16 are located
within an outline of the proof mass 14 to save space. This allows
the proof mass 14 to be made larger than would be the case if the
springs 16 were located outside the proof mass outline as in FIG. 1
when the size of the substrate 12 is fixed, or alternately allows a
smaller size substrate 12 to be used when the size of the proof
mass 14 is fixed.
Underlying the substrate 12 in FIG. 4 is a submount 50 which is
schematically shown in plan view in FIG. 5. The submount 50, which
can be the same size as the substrate 12, includes a pair of
electrodes 20, which upon actuation of the device 10, electrically
contact a metallization 22 located on an underside of the proof
mass 14.
FIGS. 6A and 6B are schematic cross section views of the MEM
acceleration switch 10 of FIGS. 4 and 5 taken along the section
line 2--2 in FIG. 4. FIG. 6A shows the device 10 in a rest
position; and FIG. 6B shows the device 10 in an actuated position
in response to a sensed acceleration component A above the
threshold level.
In the rest position in FIG. 6A, the proof mass 14 is suspended
from the substrate 12 by a plurality of folded springs 16 which are
spaced around a periphery of the rest mass 14 as shown in FIG. 4.
In FIG. 6B, the device 10 is actuated by an acceleration component
A which results in the proof mass 14 being urged into contact with
the electrodes 20 thereby providing a switch closure which can be
used to indicate the occurrence of the acceleration component A,
and that the acceleration component A is above the threshold
value.
It is possible to electrically latch the device 10 of FIG. 4, or
any of the other examples of the MEM electrical switch 10 described
herein, in a closed (i.e. actuated) position by providing a
latching electrode 52 on the submount 50 or on the package 24 (e.g.
by using a metal portion of the package 24 which can be
electrically grounded). The latching electrode 52, which is shown
in more detail in FIG. 5, can act in combination with the
metallization 22 on the proof mass 14 to form an electrical latch
which provides an electrostatic force of attraction that can
maintain the switch 10 in the closed position after occurrence of
the acceleration event above the threshold value. This can be done
in several ways. For example, a relatively high resistance
electrical connection can be made through the springs 16 to the
proof mass 14 and metallization 22 to maintain the metallization 22
at a predetermined electrical potential (e.g. ground electrical
potential). A different electrical potential can be maintained on
the latching electrode 52. The electrical potentials on the
electrode 52 and the metallization 22 can be selected so that an
electrostatic force of attraction in the rest position is much
smaller than a restoring force provided by the springs 16 so that
the proof mass 14 will not be electrostatically moved downward and
latched against the electrodes 20 without the occurrence of an
acceleration component A of a predetermined magnitude. Once the
acceleration component A is sensed by the switch 10 and the proof
mass 14 is urged downward thereby, the electrostatic force of
attraction, which increases as the inverse square of the distance
between the metallization 22 and the latching electrode 52, will be
sufficient to electrically latch the proof mass 14 in the actuated
position. This electrical latching is advantageous to reduce any
chatter (i.e. contact bounce) in the switch 10 due to a sudden or
changing acceleration component A, and is also advantageous to
maintain the switch 10 in a closed position when this is needed
after the occurrence of the acceleration event.
Another way that latching of the device 10 in FIG. 4 can be
performed is by utilizing an electrical potential provided by the
electrodes 20 in combination with a different electrical potential
on the latching electrode 52. In this mode of operation, the
metallization 22 can be left floating in the rest position, or
alternately connected to a ground electrical potential through a
relatively high-resistance current path through the springs 16 to
the substrate 12 (e.g. by using a lower doping level of for the
substrate 12, springs 16 and proof mass 14). The latching electrode
52 can also be maintained at ground electrical potential. Once the
acceleration component A is sensed and the proof mass 14 and
metallization 22 are urged into contact with the electrodes 20, an
electrical potential present on one or both of the electrodes 20
will be electrically connected to the metallization 22, and this
can result in a potential difference between the metallization 22
and the latching electrode 52 which, in turn, can generate the
electrostatic force of attraction needed to latch the device 10 in
the closed position.
The second example of the MEM acceleration switch 10 in FIG. 4 can
be fabricated as described hereinafter with reference to FIGS. 7A
7G.
In FIG. 7A, the starting point for fabrication of the second
example of the MEM acceleration switch of FIG. 4 is a conventional
silicon-on-insulator (SOI) substrate 12 which comprises a
monocrystalline silicon body 54, an insulating layer 56 overlying
the body 54, and a monocrystalline silicon layer 58 overlying the
insulating layer 56. The monocrystalline silicon body 54 can be,
for example, 400 500 .mu.m thick, with the insulating layer 56 and
the monocrystalline silicon layer 58 each being up to a few tens of
microns thick (e.g. 1 50 .mu.m). The monocrystalline silicon body
54 and the silicon layer 58 can both be doped for electrical
conductivity (e.g. n-type or p-type doped to 10.sup.15 10.sup.18
cm.sup.-3), with an exact doping level depending upon a required
electrical resistivity for the substrate 12, springs 16 and proof
mass 14. The insulating layer 56 generally comprises silicon
dioxide. The use of a SOI substrate 12 is advantageous in that the
springs 16 can be formed without any residual stress, and since
fabrication of the MEM acceleration switch 10 can be
simplified.
In FIG. 7B, the monocrystalline silicon layer 58 is patterned to
provide a plurality of openings 62 through the layer 58 to the
underlying insulating layer 56. This defines the shape of each
spring 16 which will be formed from the layer 58 and also separates
a portion of the layer 58 which will form a part of the proof mass
14 from another portion of the layer 58 which will remain a part of
the substrate 12. The patterning of the monocrystalline silicon
layer 58 can be performed by reactive ion etching using a
photolithographically-defined etch mask which is not shown in FIG.
7B. Additional openings 62 can be etched through the layer 58 and
through the insulating layer 56 and filled with deposited metal or
polysilicon to form vias (not shown) for electrically connecting
the metallization 22 to the substrate 12.
In FIG. 7C, the body 54 of the SOI substrate 12 can be patterned to
provide a cavity 36 to define the shape of the proof mass 14 being
formed from the substrate 12. This can be done by etching from a
backside of the substrate 12 using a DRIE step as described
previously with reference to FIG. 3F, with the etching being
terminated upon reaching the silicon dioxide insulating layer 56.
The cavity 36 can be extended to etch through the body 54 beneath
each spring 16, if desired. However, this is generally not needed
since the springs 16 overlie the proof mass 14 and will bend away
from the proof mass 14 as the proof mass 14 is moved downward in
response to a sensed acceleration. In the event of an upward
movement of the proof mass 14, the springs 16 can act as stops to
limit the upward movement. Additional stops 18 can also be
optionally formed from the monocrystalline silicon layer 58 during
patterning of the layer 58, with the stops 18 being attached to the
substrate 12 and overlying the proof mass 14 as schematically
illustrated in FIG. 1.
In FIG. 7D, portions of the silicon dioxide insulating layer 56 are
removed underneath the springs 16 and at the locations of each
cavity 36 to free up the proof mass 14 and the springs 16 for
movement. This can be done using a selective wet etchant comprising
HF as described previously with reference to FIG. 3G. Etching of
the insulating layer 56 can be timed to completely remove the
silicon dioxide underneath the springs 16 and proximate to the
cavities 36, with the insulating layer 56 elsewhere being left
largely intact.
In FIG. 7E, an underside major surface of the proof mass 14 can be
metallized. This can be done, for example, by evaporation or
sputtering through a shadow mask as previously described with
reference to FIG. 3H, with the metallization 22 comprising platinum
or gold. The thickness of the metallization 22 on the underside
major surface can be up to a few hundred nanometers thick, and the
metallization 22 preferably covers a majority of the surface area
of the underside major surface.
In FIG. 7F, the submount 50 can be metallized to form the
electrodes 20 and 52 thereon, and also to form electrical wiring 64
for connecting the electrodes 20 and 52 to contact pads (not
shown). The submount 50 can comprise, for example, a ceramic,
substrate having a thickness of about 0.5 1 millimeter, with
lateral dimensions of the submount 50 being the same or larger than
the lateral dimensions of the silicon substrate 12. A few hundred
nanometers thickness of metal (e.g. gold) can be deposited on the
submount 50 by evaporation or sputtering to form the electrical
wiring 64 and the latching electrode 52. The electrodes 20 can
comprise additional metal which is built up on the submount 50 to a
predetermined height (e.g. 60 .mu.m). This can be done, for
example, by a coining process whereby a quantity of gold is
ball-bonded or bump-bonded onto the electrical wiring 64 at the
location where the electrodes 20 are to be formed. The bonded gold
can then being coined (i.e. stamped or molded) into shape to
provide a precise height for each electrode 20. Alternately, the
electrodes 20 can be formed by electroplating after providing a
mask over the submount 50 with openings at the locations where the
electrodes 20 are to be formed. When the metallization 22 comprises
gold, the electrodes 20 or the metallization 22 can be optionally
coated with a layer of TiN on the order of 100 nanometers thick to
mitigate any gold--gold microwelding upon contacting of the
metallization 22 with the electrodes 20.
In FIG. 7G, the substrate 12 can be attached to the submount 50
using an intervening spacer 60 which can comprise an
electrically-insulating material such as a ceramic. The spacer 60
preferably has a thickness which provides a predetermined spacing
between the metallization 22 and the electrodes 20. This spacing
can be, for example, in the range of 20 200 .mu.m. An opening 66 is
provided through the spacer 60 centered about the proof mass 14 to
allow for movement of the proof mass 14. The opening 66 can be
formed, for example, by laser machining. The substrate 12, spacer
60 and submount 50 can all be permanently bonded together to form
the completed MEM acceleration switch 10 which can be packaged in a
hermetically-sealed conventional semiconductor header package, or
in a low-temperature co-fired ceramic (LTCC) package. Bonding of
the substrate 12, spacer 60 and submount 50 can be performed using
an adhesive (e.g. epoxy) or by LTCC processing as known to the art.
When LTCC processing is used, a ceramic seal ring can be provided
to bond a ceramic cover onto the submount 50 to package the MEM
acceleration switch 10.
FIG. 8 schematically illustrates in plan view a third example of a
MEM acceleration switch 10 according to the present invention. The
device 10 of FIG. 8 utilizes a plurality of arcuate springs 16 to
suspend the proof mass 14 from the substrate 12, with each spring
16 being attached at one end thereof to the substrate 12 and at the
other end thereof to the proof mass 14. The arcuate springs 16 are
preferably made much stiffer in a lateral direction in the plane of
the substrate 12 than in a vertical direction perpendicular to the
plane of the substrate 12. The stiffness of the arcuate springs 16
in the lateral direction, however, is much smaller than that of the
devices of FIGS. 1 and 4 which utilize folded springs 16. A very
large acceleration component (e.g. due to shock) in the devices of
FIGS. 1 and 4 can lead to a buckling (i.e. breaking) of the folded
springs 16; whereas the arcuate springs 16 in the example of FIG. 8
are able to bend laterally so that they are much less susceptible
to breaking. This in-plane movement allowed by the arcuate springs
16 in the example of FIG. 8 will not result in actuation of the MEM
acceleration switch 10, but is merely provided to mitigate the
possibility for breakage of the springs 16. A plurality of lateral
stops 68 can also be provided in the device 10 of FIG. 8 to limit
an extent of the lateral movement of the proof mass 14 so that the
proof mass 14 does not bang against the arcuate springs 16 when a
large lateral acceleration component is experienced. The lateral
stops 68 can extend outward from a sidewall of the cavity 36 as
shown in FIG. 8, with the lateral stops 68 being formed during one
or more DRIE steps used to form the cavity 36 so that the lateral
stops 68 extend partway or entirely through the thickness of the
monocrystalline silicon body 54.
The device 10 of FIG. 8 can be fabricated as previously described
with reference to FIGS. 3A 3I with the springs 16 being formed of
polysilicon, or as previously described with reference to FIGS. 7A
7G with the springs 16 being formed of monocrystalline silicon.
Operation of this example of the MEM acceleration switch 10 is
similar to that described previously for the first and second
examples of the switch 10, with the proof mass 14 being urged
downward in response to an acceleration component A of a
predetermined magnitude in a direction substantially perpendicular
to the plane of the substrate 12. When the acceleration exceeds a
predetermined threshold value, a metallization 22 located on an
underside of the proof mass 14 will contact one or more electrodes
20 and result in a switch closure.
The matter set forth in the foregoing description and accompanying
drawings is offered by way of illustration only and not as a
limitation. The actual scope of the invention is intended to be
defined in the following claims when viewed in their proper
perspective based on the prior art.
* * * * *